For months after the SNP defied electoral arithmetic to secure majority status at Holyrood in 2011, its control of the Scottish political landscape seemed absolute. With his unionist opponents humiliated, Alex Salmond was effectively free to run things on his own terms.

Recently, however, a series of badly executed policy U-turns and poorly handled referendum controversies – over Nato membership, EU legal advice and Scotland’s currency options – has taken the wind out of the nationalists’ sails. Now another controversy is stirring as the party’s commitment to cut corporation tax comes under increasing scrutiny.

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